= Hydrostatic internal-wave propagation geometry
{title2=$\mathbf c_g=(\omega/k,-\omega/m)$}
For a positive-frequency non-rotating <internal gravity wave> with $\omega=N|k|/|m|$, differentiation gives <group velocity> $(\omega/k,-\omega/m)$. Its <wavevector> $(k,m)$ is orthogonal to this vector, so <energy> travels along constant-phase lines and perpendicular to normal <phase velocity>. The horizontal crest-intersection speed $\omega/k$ is distinct from the horizontal component $\omega k/(k^2+m^2)$ of that normal <phase velocity>. The <hydrostatic approximation> requires $|k/m|\ll1$.
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